Bones consist of cancellous bone covered by a thin layer of cortical bone as illustrated in FIG. 1. Cancellous bone is a sponge-like bone structure which is less dense, softer, and weaker when compared to cortical bone. Bone screws are utilized in surgery typically to stabilize and fix bone segments or to use as an anchor site within the bone. Most commonly, the screws are advanced through the outer cortical wall and anchored into the cancellous bone within. However, bi-cortical fixation can be used to achieve greater purchase, as the screw is fixed within the stronger cortical bone at two separate points, the proximal and distal ends of the screw. Doing so increases the screw's pull out strength, which may be desirable at higher load levels, such as in the lower lumbar and sacrum of the spine.
Safely achieving bi-cortical screw purchase is often difficult however. In the human vertebrae for example, the goal of bi-cortical pedicle screw fixation is to reach and thread the lead end of the screw into the anterior cortical wall. If the tip of the screw or associated instrumentation is advanced too far beyond the anterior cortical wall, the vital tissues that reside adjacent the anterior wall of the vertebrae, the great vessels for example, may be put at risk. Even with utilization of intraoperative fluoroscopy, safely gauging a screw's position can be difficult. As illustrated in FIG. 2, the curvature of the anterior cortical wall of the vertebral body may cause difficulty correctly determining the position of a screw from a lateral view, such that in the lateral fluoroscope image the distal end of the screw may appear to be contained within the vertebra since the final depth of the distal end may be less than the vertebral depth at the anterior most portion (FIG. 2A). However, the actual screw position, FIG. 2B, is such that the distal end of the screw protrudes beyond the anterior cortical wall but at a position where the depth of the wall is less than the greatest depth near the center.
Current methods of bi-cortical screw fixation rely heavily on surgeon feel when forming and/or tapping the pilot hole through the vertebral body and/or during screw insertion. Thus, a need exists for instruments and methods to facilitate bi-cortical implantation of bone anchors.